In recent years, a large number of portable electronic devices such as camcorders, digital still cameras, cellular phones, personal digital assistances and laptop computers have been emerged, and an attempt to reduce the size and the weight of them has been made. Accordingly, the development of lightweight secondary batteries capable of obtaining a high energy density as power sources for the electronic devices have been promoted. Among them, a lithium-ion secondary battery using a carbon material for an anode, a composite material of lithium (Li) and a transition metal for a cathode and a carbonate for an electrolytic solution has been widely put to practical use, because the lithium-ion secondary battery can obtain a larger energy density than a lead-acid battery and a nickel-cadmium battery in related arts.
Moreover, recently as the performance of portable electronic devices is enhanced, a further improvement in capacity is desired, and it is considered to use tin, silicon or the like as an anode active material instead of a carbon material (for example, refer to Patent Literature 1). It is because the theoretical capacity of tin, 994 mAh/g, and the theoretical capacity of silicon, 4199 mAh/g are much larger than the theoretical capacity of graphite, 372 mAh/g, so an increase in capacity can be expected.
However, a tin alloy or a silicon alloy into which lithium is inserted has high activity, so there is an issue that an electrolytic solution is easily decomposed, and lithium is inactivated. Therefore, when charge and discharge are repeated, charge-discharge efficiency declines, thereby sufficient cycle characteristics cannot be obtained.
Therefore, it is considered to form an inert layer on a surface of an anode active material, and, for example, it is proposed to form a coating film of silicon oxide on a surface of an anode active material (for example, refer to Patent Literature 2). On the other hand, it is considered that when the thickness of the coating film of silicon oxide is increased, reaction resistance is increased, thereby cycle characteristics become insufficient (for example, refer to Patent Literature 3). In the past, such a coating film of silicon oxide is formed by air oxidation or a vapor-phase method.    [Patent Literature 1] U.S. Pat. No. 4,950,566    [Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2004-171874    [Patent Literature 3] Japanese Unexamined Patent Application Publication No. 2004-319469